For most of the twentieth century the removal of carbon dioxide from other gases has been the subject of considerable industrial research. Many processes have been developed for this purpose. The most common methods for separating out carbon dioxide involve selective adsorption, often with reversible chemical reactions of the carbon dioxide with chemicals in the sorbent. Well known methods involve bubbling a gas stream containing carbon dioxide through caustic liquids, such as alkanol amine solutions or solutions containing soda, ammonia and/or carbonates, that adsorb CO.sub.2 through the formation of metal carbonate salts, such as Na.sub.2 CO.sub.3. A process of this type is described in U.S. Pat. No. 1,831,731, J. Al, (1931) which discloses reacting CO.sub.2 with K.sub.2 CO.sub.3 in the presence of water to form KHCO.sub.3. The alkali carbonate can be regenerated with ammonia. These liquid systems are costly, tend to entrain liquid in downstream equipment, generally require high maintenance, and are practical only at moderate temperatures, e.g. 20 to 50.degree. C.
A number of solid state adsorbents have been developed to avoid the above-mentioned problems associated with liquid systems. For example, U.S. Pat. No. 3,141,729, Clarke et al., (1964) describes removing CO.sub.2 from an atmosphere with a cogel of a lithium or divalent metal oxide with a trivalent metal oxide, such as MgO. Al.sub.2 O.sub.3. Also, U.S. Pat. No. 5,520,894, Heesink et al., (1996) discloses removing CO.sub.2 from a hot gas stream such as flue gas with a solid absorbent of CaO, MgO or CaCO.sub.3. MgO. These systems require elaborate operating procedures, such as preprocessing the gas streams to remove competitive species, or extreme conditions of regeneration.
Several patents describe the removal of CO.sub.2 from gas streams with adsorbents supported on alumina. U.S. Pat. No. 3,511,595, Fuchs, (1970) discloses the removal of CO.sub.2 by reaction with an alkali metal carbonate coated on or impregnated in a high surface area carrier such as alumina. U.S. Pat. No. 3,865,924, Gidaspow et al., (1975) describes removing CO.sub.2 with an alkali metal carbonate ground together with alumina. U.S. Pat. No. 4,433,981, Slaugh et al., (1984) discloses removing CO.sub.2 with a calcined oxide or decomposable salt of an alkali metal or alkaline earth metal impregnated on a porous alumina support. U.S. Pat. No. 4,493,715, Hogan et al., (1985) discloses removing CO.sub.2 from an olefin stream using a calcined alkali metal compound on alumina. In each of these patents, regeneration of the adsorbent is accomplished by heating in a temperature swing operation.
Although not part of the CO.sub.2 adsorption art, a description of complex or double salts of magnesium carbonate is given in the text, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, J. W. Mellor, John Wiley & Sons, N.Y., Vol. 4, pp. 367-376 (1960). This reference discloses that heating crystalline double salts of alkali metal and magnesium carbonate, e.g. K.sub.2 Mg(CO.sub.3).sub.2.4H.sub.2 O or KHMg(CO.sub.3).sub.2.H.sub.2 O, results in evolution of CO.sub.2 leaving a mixture of MgO and K.sub.2 CO.sub.3. Although these carbonate double salts containing alkali metals and magnesium in stoichiometric proportions have been known for several years, there is no known reference to non-stoichiometric double salts nor to the study or use of this family of materials as CO.sub.2 adsorbents.
U.S. Pat. No. 5,454,968, Nalette et al. (1995) which, while not referring to their compositions as double salts, describes the formation of pastes of mixtures of metal carbonate and alkali metal carbonate that can be formed into flat sheets which are used to sorb carbon dioxide from gas streams flowing through the sheets. The pastes are prepared by forming an aqueous solution of alkali metal carbonate, e.g. K.sub.2 CO.sub.3, which is blended with a powder of metal carbonate, preferably silver carbonate. Other metals mentioned are zinc and magnesium. The paste is then formed into a flat sheet, constrained between screens, and heated to drive off water and to convert the silver carbonate to silver oxide. The CO.sub.2 and H.sub.2 O are said to react with K.sub.2 CO.sub.3 during operation to form KHCO.sub.3 which then reacts with AgO to form AgCO.sub.3 and K.sub.2 CO.sub.3 plus H.sub.2 O. Regeneration is accomplished by heating, e.g. to 160-220.degree. C., to liberate CO.sub.2 from the silver carbonate. The maximum operating temperature is said to be 250.degree. C. The sorbents of Nalette et al. have been found in practice not to be totally satisfactory, particularly in high temperature operations where regeneration of the adsorbent must be accomplished by thermal swings rather than by pressure swings. Therefore, further improvements in CO.sub.2 adsorbents is highly desirable.